Toner concentration control apparatus

ABSTRACT

A toner concentration control apparatus includes a circuit responsive to the cummulative total charge of electrostatic images and/or toner images which pass a predetermined position of a path for producing a signal which is an accurate measure of the quantity of toner which will and/or has been applied to such images.

United States Patent [191 Altmann 1 Dec. 18, 1973 1 TONER CONCENTRATIONCONTROL APPARATUS [75] Inventor: Conrad Altmann, Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, N.Y.

[22] Filed: Aug. 14, 1972 [21] App1. No.: 280,396

[52] US. Cl 118/8, 118/637, 324/72 [51] Int. Cl G03g 13/16 [58] Field ofSearch 118/7, 8, 637, DIG. 24;

117/175; 346/74 ES; 355/3, 3 DD; 324/32, 72

[56] References Cited UNITED STATES PATENTS 10/1960 Giamo, Jr. 118/637UX 2/1963 Codichini 6/1963 Sneliing 324/32 X 3,161,882 12/1964 Muilin117/175 X 3,348,522 10/1967 Donohue [18/637 X 3,367,307 2/1968 Lawes eta1 118/7 X 3,406,334 10/1968 Marquart et a1 324/72 X 3,449,658 6/1969Robinson et a1 324/72 X 3,674,353 7/1972 Trachtenberg 355/3 DD PrimaryExaminer-Morris Kaplan AttorneyW. 1-1. .1. Kline et a1.

[57] ABSTRACT A toner concentration control apparatus includes a circuitresponsive to the cummuiative total charge of electrostatic imagesand/or toner images which pass a predetermined position of a path forproducing a signal which is an accurate measure of the quantity of tonerwhich wi11 and/or has been applied to such images.

8 Claims, 3 Drawing Figures sir/9.203

PATENTED DEC 18 I975 SHEET 10F 2 Pmmmuu: 1 a mu 3.779.203

SHIT 2 [IF 2 68 FROM 1+ 64 a 7 4 THRESHOLD To DECTECTOR 52 P 76 x 75DELAY CIRCUIT 34 FIG. 3

TONER CONCENTRATION CONTROL APPARATUS BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to electrographicdevelopment apparatus, and more particularly, to toner concentrationcontrol apparatus for maintaining the relative con centration of tonerin a mixture of toner and carrier components at a level wherein copiesof uniformly good density are consistently producable.

2. Description of the Prior Art In electrographic apparatus, anelectrostatic image, formed on the surface of a drum or web, isdeveloped by application of finely divided toner particles thereon toform a toner image. In certain electrographic apparatus, toner imagesare formed from electrostatic images by brushing a developer mixture offerromagnetic carrier particles and much smaller toner particles(typically suitably pigmented or dyed resin-based particles) across theelectrostatic images. The contact of the ferromagnetic particles withthe toner particles charges the toner particles by triboelectrificationto a polarity needed so that the toner particles are attracted to theelectrostatic images for toning. In this process, toner particles aredepleted from the developer mixture, requiring subsequent replenishmentto avoid a gradual reduction in density of the toner images. Tonerreplenishment is accomplished by several different types of apparatus.In one type, a given amount of toner is added to the mixture after agiven number of copies is made. This approach is acceptable providingthe amount of toner used for each copy is reasonably predictable. Inapparatus which do not tone broad, solid areas, however, such as manymagnetic brush developing and cascade toning apparatus, the amount oftoner used in any copy of group of copies can vary substantially. Forthis reason, toner concentration monitors have been designed whichautomatically add toner according to the results .of a monitoringprocess; see for example U.S. Pat. Nos. 2,956,487 to Giamo; 3,094,049 toSnelling; 3,227,549 to Ullrich, 3,233,781 to Grubbs; 3,399,652 toGauron; and 3,409,901 to Dost et a1.

Certain toner concentration monitors examine the quality of a tonerimage before adding toner. These apparatus suffer from the disadvantagethat toner is not added until only after copy quality is degraded.

In certain optical monitoring apparatus a reduction in the relativetoner concentration in the mixture below a desired level can be detectedin the mixture of toner and carrier before image degradation. (See forexample U.S. Pat. Nos. 3,233,781 to Grubbs and 3,409,901 to Dost etal).Such optical apparatus have not proven entirely satisfactory inoperation, especially over extended periods of time. This may beattributed, at least in part, to the lighting source utilized forilluminating the developer mixture which may be a luminous energy sourceof relatively low output, unstable intensity and short life, and furtherto the fact that spectral characteristics of light from the source maybe closely akin to the background light, thereby making backgrounddiscrimination difficult. Also contributing to the instability of suchdevices is the fact that the photoconductive cell used for detectingdeveloper reflectance may be sensitive to temperature and humidityvariations of the type encountered in normal machine operation, or mayexhibit an undesirable hysteresis or light history effect. Moreover,when photocells are confronted with the interference of airborne tonerparticles they often provide inaccurate readings. In general, opticalmonitoring apparatus require frequent cleaning of components to assurefreedom from erroneous measurements.

Another approach is set forth in commonly assigned copending U.S. PatentApplication Ser. No. 158,174, filed July l, 1971, now U.S. Pat. No.3,674,353, entitled, TONER CONCENTRATION CONTROL APPA- RATUS in the nameof William Trachtenberg. This invention pertains to an open loop tonerconcentration monitor wherein monitoring is achieved by sensing thecharge of an image before and after development. The monitor produces ananalog signal representative of the difference between, before and afterimage charges. Although such an approach can perform satisfactorily, itis difficult in practice to correlate the analog signal in terms oftoner lost to the development station.

SUMMARY OF THE INVENTION An object of the present invention then is toimprove electrographic development by providing an apparatus forautomatically maintaining the toner concentration in an electrographicdeveloper at a substantially constant level at which high quality tonerimages can be consistently produced.

Another object of the invention is to provide an inexpensive, yet highlyaccurate and reliable, apparatus for automatically controlling the levelof toner concentration in an electrographic developer.

Another object of the invention is to provide an improved tonerconcentration control apparatus which does not require the need formonitoring the mixture of toner and carrier components.

In accordance with the invention, it has been determined that themagnitude of the total charge of an electrostatic latent image isdirectly related to the amount of toner which will be attracted to theelectrostatic image and lost to the development station.

In the disclosed apparatus, a toner concentration control apparatusincludes means for producing a signal which is a function of thecummulative total charge of electrostatic images passing a predeterminedposition along a path. The total charge of each electrostatic image isrelated to the amount of toner which is required to develop such image.The apparatus further includes means responsive to a particularmagnitude of said signal for causing toner to be added to the mixture toincrease the relative concentration of the toner component and obtain adesired relative concentrations of mixture components.

Other objects of the invention and its various advantages will becomeapparent from the ensuing detailed description of the preferredembodiment shown below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic vertical sectionof an apparatus embodying the invention; and

FIGS. 2 and 3 are circuit diagrams partially in schematic and partiallyin block form which may be utilized with the apparatus shown in FIG. 1.-

DESCRIPTION OF THE PREFERRED EMBODIMENT For a general understanding ofan electrographic reproducing or copier apparatus wherein the inventionhas particular utility, reference is made to FIG. 1

wherein various stations of an electrophotographic ap-' member 16, shownas a web, to selectively dissipate change and form an electrostaticimage. The web had previously been uniformly charged at a chargingstation 20. The photosensitive member 16 includes a photoconductivelayer with a conductive backing. The photoconductive layer may comprise,for instance, a heterogeneous mixture of a bisphenol A-polycarbonatebinder, a triarylmethane organic photoconductor and a pyryliumsensitizing dye. The apparatus further includes a development station 22at which the electrostatic image is contacted with finely dividedcharged toner particles that adhere' to the photoconductive layer in aconfiguration defined by the electrostatic image; a transfer station 24in which toner particles are transferred in the image configuration to areceiving surface of a copy sheet 26 on which it can be subsequentlypermanently fused; and a cleaning station (not shown) inwhich residualtoner is removed from the photoconductive layer so that it can bereused. In certain known modifications of this same system, one or moreof these stations may be eliminated. For a more complete description ofan electrographic apparatus reference may be made to commonly assignedcopending patent application Ser. No. 191,304 filed Sept. 21, 1971, nowU.S. Pat. No. 3,698,805, entitled CON TROL APPARATUS FOR ELECTROPI-IOTO-GRAPHIC APPARATUS to Hickey et al. An example of an exemplarydevelopment station is set forth in U.S. Pat. No. 3,543,720 to Drexler.The development station 22 shown is as a magnetic brush-type apparatuswhich includes a cylindrical brush member 30 which is rotated in acounterclockwise direction as shown by the arrow, in a magnetic fieldproduced by magnets (not shown). The brush member 30 is effective totransport a portion of a mixture 33 of ferromagnetic carrier particlesand toner particles from a developer reservoir 34 which is a conductivemember and acts as an electrode across the surface of the web 16 suchthat toner particles will contact a portion of and then remain oncharged image areas on the surface of the web 16. Since a mixture havinga high concentration toner would tend to clog, a continuous controlmixing paddle-wheel 35 has been provided to uniformly blend the mixture.

As the relative concentration of toner in the mixture decreases, theresulting developed toner images will tend to have a lower density andeventually will appear washed-out. In order to alleviate this condition,a toner concentration monitor has been provided which is shown generallyat 40 and is coupled to a charge sensor (disposed at a predeterminedposition along the web path) such as an electrometer 47 which is coupledto a source of potential shown for convenience of illustration as abattery 48. The probe 47 is also coupled to a development stationpotential control apparatus 71 (also referred to herein as an auto biassystem which is adapted to control the potential of the developmentreservoir 34 to provide the well known autobias function described indetail in U.S. Pat. No. 2,956,487. An example of an electrometersuitable for use with the invention would be an induction plate or probefabricated from a highly conductive material such as copper or aluminum,and spaced closely to the web to web to receive an induced charge as anelectrostatic image passes by. The electrometer 47 may extend over thewhole width of the area and is adapted to scan the electrostatic imageand produce an electrostatic signal which is proportional to the chargedensity of that p0rtion of the electrostatic image passing under theprobe 47. In placing the probe 47, it is necessary to take into accountthe factor that some photoconductive members exhibit a sharp reductionin conductivity for a short time period, usually less than one second,following exposure. Thus, it is desirable to displace the electrometer47 a sufficient distance from the exposure station 21 so that the effectof such conductivity reduction will have run its course before theundeveloped image arrives at the meter 47. The probe 47 is directlycoupled to the auto-bias system 71 which will be described in connectionwith FIG. 3. I

The signal produced by the probe 47 is also applied to the circuitry 50.The circuitry 50 is effective when the toner concentration of themixture-will be reduced below a desired level to actuate a solenoiddevice 52 which rotates a metering wheel 57 releasing a predeterminedincrement of toner from a toner container 56. Such toner increment,under the influence of gravity, falls into the reservoir 34 wherein thepaddle-wheel 35 uniformly blends it into the developer mixture 33,thereby returning the relative concentrations of toner and carriercomponents to a desired level. One of the advantages obtainable with thetoner control apparatus 40 is that any conventional toner 'replenishermechanism can be actuated by the aforementioned output signal, such asfor example that disclosed in U.S. Pat. No. 3,409,901.

Turning now to FIG. 2, wherein the circuit 50 for actuating the solenoid52 is shown in more detail and includes an integrator 60 shown as anoperational amplifier with a feedback capacitor 62 coupled between itsoutput and non-inverting input terminals.

The inverting input of the operational amplifier 60 is coupled toground. The output signal produced by the amplifier 60 is applied to acharging circuit comprising an adjustable resistor 64 and a chargingcapacitor 66 coupled to ground. At the electrical junction of theresistor 64 and the capacitor 66 there is provided a threshold detectorcircuit 68 which may include a conventional unijunction transistor whichfires when the capacitor 66 reaches a predetermined charge level. Atsuch time, the threshold circuit 68 will provide a signal to thesolenoid 52 causing its actuation.

Briefly, in operation, the circuit 50 functions as follows. The chargelevel of an electrostatic charge density (viz., field strength) iscontinuously sampled and the output of the operational amplifier 60provides a signal representative of the cumulative total charge level ofelectrostatic images passing by the probe 47. When a predeterminedcharge level on the capacitor 66 is reached (which may be changed byadjusting the resistance of the resistor 64) the threshold detectorcircharge on the capacitor 66 and reduce the output potential of theamplifier 60 to ground level. The relay 70 is shortly thereafterde-energized and opens contacts 70a thereby conditioning the circuit 50to repeat the above operation.

A second embodiment of the invention is shown in H6. 1 wherein a probe47 (shown in dotted lines) is disposed in a predetermined position alongthe web path disposed after the development station 22. Assuming theelectrostatic images have a negative potential, then toner imagescorresponding thereto will have positive potential. Accordingly theprobe 47 provides a similar function as the probe 47 but will provide asignal to the circuitry 50 which is representative of the charge densityof deposited toner. The integrator 60 now provides a signalrepresentative of the total change of toner and the threshold detector68 will be actuated when a predetermined amount of toner has been lostto the reservoir 34.

The autobias system 71 is shown in FIG. 3 and includes a preamplifier 72which receives a signal directly from the probe 47 and amplifiers same.The preamplifier 72 also provides an isolation function. A conventionaldelay circuit 74 receives an input from the amplifier 72 and is adaptedto delay the output signal of the amplifier 72 for a time periodsufficient to permit an electrostatic image sampled by the probe 47 toadvance over the development station for toning. An example of such acircuit is set forth in U. S. Pat. No. 3,61 1,982. The output of thedelay circuit 74 is applied through an adjustable resistor 75 to a poweramplifier 76 which adjusts the output signal to a potential levelsuitable for use and applies it to the reservoir or electrode 34.

It will be appreciated by those skilled in the art that the subjectinvention has utility in a number of electrographic reproductionsystems, including xeroprinting, thermoxerography, and xerothermography,and in no way is dependent upon the physical form of the developer (viz.liquid or particulate) or the manner in which it is applied to theelectrostatic image to render it visible.

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1 claim:

1. For use in electrographic apparatus having a member charged with apattern forming an electrostatic image, developing means for contactingsuch electrostatic images with a mixture of toner and carrier componentsto form toner images corresponding to the electrostatic images andactuable toner replenishment means effective when actuated for addingtoner to the mixture to compensate for toner depletion resulting fromforming the toner images, an improved toner replenishment controlapparatus comprising:

a. means for sensing the charge density on electrostatic images prior totoning and for producing a signal the magnitude of which isrepresentative of the total cummulative charge thereof passing apredetermined portion; and

b. means responsive to a predetermined magnitude of said signal foractuating the toner replenishment means to cause the addition of tonerto the mixture.

2. In electrographic apparatus in which a surface carrying anelectrostatic charge pattern is contacted by a developer mixture ofcarrier particles and electrostatically attractable toner particles toform a toner image on the surface according to the pattern, a tonerconcentration apparatus comprising:

a. first means for sensing the electric field strength associated withan electrostatic charge pattern to provide a signal representativethereof; and

b. second means responsive solely to said signal for indicating theamount of toner which will be depleted by producing a second signal themagnitude of which is a function of the amount of toner which will bedeposited on the surface.

3. The toner concentration apparatus as set forth in claim 2 including:

c. a container for storing toner particles;

(1. actuable means effective when actuated for causing toner from saidcontainer to be delivered to the mixture; and

e. said second means including means responsive to a particularmagnitude of said second signal for actuating said actuable means.

4. The invention as set forth in claim 3 wherein said second meansincludes an integrator circuit.

5. For use in electrographic apparatus having a photoconductive memberadapted to form electrostatic images, developing means for contactingsuch electrostatic images with a mixture of toner and carrier componentsto form toner images corresponding to the electrostatic imagesrespectively, and actuable toner replenishment means effective whenactuated for adding toner to the mixture to compensate for tonerdepletion resulting from forming the toner images, an improved tonerreplenishment control apparatus comprising:

a. means for sensing the charge density on toner images and forproducing a signal the magnitude of which is representative of the totalcummulative charge thereof passing a predetermined portion; and

b. means responsive to a predetermined magnitude of said signal foractuating the toner replenishment means to cause the addition of tonerto the mixture.

6. In electrographic apparatus in which a surface car- 5 rying anelectrostatic charge pattern is contacted by a developer mixture ofcarrier particles and electrostatically attractable toner particles toform a toner image on the surface according to the pattern, a toner concentration apparatus comprising:

a. first means for sensing the electric field strength associated with atoner image and providing a signal representative thereof; and

b. second means responsive solely to said signal for indicating tonerdepletion by producing a second signal the magnitude of which is afunction of the amount of toner deposited on the toner image.

7. The toner concentration apparatus as set forth in claim 6 including:

c. a container for storing toner particles;

d. actuable means effective when actuated for causing toner from saidcontainer to be delivered to the mixture; and

e. said second means including means responsive to a particularmagnitude of said second signal for actuating said actuable means.

8. The invention as set forth in claim 7 wherein said second meansincludes an integrator circuit.

k OK I.

1. For use in electrographic apparatus having a member charged with apattern forming an electrostatic image, developing means for contactingsuch electrostatic images with a mixture of toner and carrier componentsto form toner images corresponding to the electrostatic images andactuable toner replenishment means effective when actuated for addingtoner to the mixture to compensate for toner depletion resulting fromforming the toner images, an improved toner replenishment controlapparatus comprising: a. means for sensing the charge density onelectrostatic images prior to toning and for producing a signal themagnitude of which is representative of the total cummulative chargethereof passing a predetermined portion; and b. means responsive to apredetermined magnitude of said signal for actuating the tonerreplenishment means to cause the addition of toner to the mixture.
 2. Inelectrographic apparatus in which a surface carrying an electrostaticcharge pattern is contacted by a developer mixture of carrier particlesand electrostatically attractable toner particles to form a toner imageon the surface according to the pattern, a toner concentration apparatuscomprising: a. first means for sensing the electric field strengthassociated with an electrostatic charge pattern to provide a signalrepresentative thereof; and b. second means responsive solely to saidsignal for indicating the amount of toner which will be depleted byproducing a second signal the magnitude of which is a function of theamount of toner which will be deposited on the surface.
 3. The tonerconcentration apparatus as set forth in claim 2 including: c. acontainer for storing toner particles; d. actuable means effective whenactuated for causing toner from said container to be delivered to themixture; and e. said second means including means responsive to aparticular magnitude of said second signal for actuating said actuablemeans.
 4. The invention as set forth in claim 3 wherein said secondmeans includes an integrator circuit.
 5. For use in electrographicapparatus having a photoconductive member adapted to form electrostaticimages, developing means for contacting such electrostatic images with amixture of toner and carrier components to form toner imagescorresponding to the electrostatic images respectively, and actuabletoner replenishment means effective when actuated for adding toner tothe mixture to compensate for toner depletion resulting from forming thetoner images, an improved toner replenishment control apparatuscomprising: a. means for sensing the charge density on toner images andfor producing a signal the magnitude of which is representative of thetotal cummulative charge thereof passing a predetermined portion; and b.means responsive to a predetermined magnitude of said signal foractuating the toner replenishment means to cause the addition of tonerto the mixture.
 6. In electrographic apparatus in which a surfacecarrying an electrostatic charge pattern is contacted by a developermixture of carrier particles and electrostatically attractable tonerparticles to form a toner image on the surface according to the pattern,a toner concentration apparatus comprising: a. first means for sensingthe electric field strength associated with a toner image and providinga signal representative thereof; and b. second means responsive solelyto said signal for indicating toner depletion by producing a secondsignal the magnitude of which is a function of the amount of tonerdeposited on the toner image.
 7. The toner concentration apparatus asset forth in claim 6 including: c. a container for storing tonerparticles; d. actuable means effective when actuated for causing tonerfrom said container to be delivered to the mixture; and e. said secondmeans including means responsive to a particular magnitude of saidsecond signal for actuating said actuable means.
 8. The invention as setforth in claim 7 wherein said second means includes an integratorcircuit.